1. Field of the Invention
This invention relates to planographic printing plates. More particularly, this invention relates to negative-working, waterless planographic printing plates and their use in lithographic printing.
2. Description of Related Art
A negative-working, dry or waterless, planographic printing plate is one which is imaged with a negative original and used in lithographic printing that does not require dampening with water. A typical plate of this type consists of (1) a silicone layer, (2) a positive working radiation sensitive imaging layer, e.g., a photosolubilizable layer, and (3) an ink receptive substrate. After exposure to actinic radiation, typically ultraviolet (UV) radiation, through a negative original, a developer penetrates the silicone layer and dissolves the areas of the radiation sensitive coating which were exposed to the radiation and those areas with the overlying areas of the silicone layer are removed from the ink receptive surface. The coatings in areas not exposed to radiation remain intact and form ink repelling areas of the plate.
Such a negative-working, dry planographic printing plate was first disclosed in U.S. Pat. No. 3,511,178 in which the radiation sensitive layer consists of a tungsten-diazonium complex. However such sensitive layers were found to suffer from poor thermal stability. U.S. Pat. No. 3,933,495 discloses a waterless plate whose light-solublizable sublayer comprises a reaction product of a hydroxy-benzophenone compound with the condensation product of p-diazo diphenylamine and formadehyde. This composition becomes more soluble in cyclohexanone upon exposure to UV light. Among other limitations, the obnoxious odor and toxicity of cyclohexanone limit commercialization of this technology.
Although naphthaquinone diazides have long been known for use in light-solubilizable compositions, their use in the light-solublizable sublayer for negative waterless plates was not realized until 1982, when U.S. Pat. No. 4,342,820 disclosed a crosslinked phenolic composition functionalized with naphthaquinone diazide species. Since the binder resin was crosslinked, naphthaquinone diazide functionality provided photo-induced swellablity instead of photo-induced solubility. More recently, U.S. Pat. No. 4,358,522 discloses plates in which the radiation layer uses 1-naphthaquinone-2-diazide-5-sulfonate of a Novolac resin that must have 44-65% esterification of the phenolic hydroxy groups and must contain less than 20% ethanol soluble fraction. This type of diazide resin requires special synthesis and purification efforts and, therefore, is expensive to prepare. In both U.S. Pat. Nos. 4,342,820 and 4,358,522, an adhesion promoter such as .gamma.-aminopropyltriethoxy silane between the silicone top layer and the radiation sensitive sublayer is essentially required and adds to the manufacturing costs. While the adhesion promoter is effective for condensation-cured silicone elastomer compositions, it is not as effective for addition-cured silicone elastomer compositions. Addition-curing and condensation curing compositions are two primary crosslinking technologies used in silicone elastomer preparation. Addition-curing involves addition of Si--H groups to vinyl groups, whereas condensation curing involves reaction of Si--OH terminated siloxane polymers with silane crosslinkers containing silicone atoms attached to a plurality of OH groups. Such silane crosslinkers usually are formed in situ via hydrolysis of acetoxy or alkoxy silanes.
U.S. Pat. No. 4,842,990 discloses a waterless planographic printing plate with an amorphous silicic acid interlayer in which solubilization is light induced by acid-catalyzed hydrolysis. This type of plate typically requires an extra step in processing, i.e., post-exposure heating. Due to non-uniformity of oven temperature, such post-exposure heating often limits consistent image reproduction.